Because poor people are not stupid. They're often actually pretty damn rational within their constraints. Smarts alone does not equip one to handle a windfall.
This is a very crucial point. The so called "stupid" financial behavior of people in (first world) poverty is anything but, it's a series of carefully calculated and measured responses to extreme inflexibility. Almost all means of establishing long term financial stability involve either a) put away some money now, wait for some time, then receive more money or b) risk losing some money for the chance to gain more money in a way that averages to a net positive over time. Neither of these are possible when you do not have flexibility, you can't afford to put away money when doing so means not eating for a week, and you can't afford to risk money for more money when coming out on the losing end of that deal means becoming homeless. You have to have a completely different approach to managing money when in poverty, and it sticks with you mentally even after escaping to a more stable situation.